Such castor assemblies are to be regarded as `consumables` in this field. In the United States alone, around two million castor assemblies are used each year. It is, consequently, important that the castor assemblies themselves are of rugged construction so as to minimise wear, that they are mounted on decks or dollies so as to minimise the likelihood of damage and, finally, that they are mounted in such a way that they can be quickly and easily replaced when worn.
In the case of dollies it is also desirable that the dollies themselves should be of lightweight construction so that they can be moved easily from place to place. Most known dollies consist of a sub-frame, provided with wheels, on which the castor assemblies are mounted. A strong and relatively heavy sub-frame is needed to support the castor assemblies but the openings defined by the sub-frame between the castor assemblies are dangerous to personnel, who can easily damage legs and ankles if walked upon and they slip into these openings. In addition, the relatively open sub-frame allows the castors to be damaged in use. It has therefore been usual to close over the sub-frame by means of cover plates which serve to protect the castor assemblies against damage and when a castor "toeplate" is fitted also protect personnel against injury.
The need for both a substantial sub-frame and cover plates to protect the castor assemblies have added to the weight of the dolly.
Decks are used not only in airport buildings and the like but also inside the cargo holds of aircraft themselves, where space and weight is at a premium. In such locations, it is essential that decks be made as shallow as possible so that they do not reduce the usable height of the cargo hold and, hence, the available storage space, any more than is necessary.